The question for the SID is a tricky one. A good friend of mine just had his 46 year old French Reims 172 being SIDed. Thanks to the good corrosion protection of the Reims production the machine had no major issue.
There was some minor corrosion starting at some of the hidden connections and beginning area corrosion at the cell surfaces under the plastic covers. They said they did not expect this for an always hangared Reims one and it is not a big issue NOW, can be fixed easily with protective coating and is not very costly, but it is about time to start fight the beginnings. Wing bolts and attachments are good to check, as well as hidden corners, where condensing water could have done something. All over, my friend said it was a good decision - especially for sell capability - and now it is for sure ready for the next 10 years.
In short: yes, it is an ADVISABLE thing to do, BUT wether the results justify a "mandatory" label, like the german LBA interprets, is more then questionable.